There are about 15,000 nuclear weapons in the world, easily enough to wipe out humanity. As president, Donald Trump will control nearly half of them.

What he’ll do with that responsibility is not at all clear. Sometimes he’s seemed like a hawk, other times a downright hippie.

“Basically the president has a hell of a lot of control over nuclear weapons,” nuclear analyst Miles Pomper of the James Martin Center for Nonproliferation Studies told BuzzFeed News. “And there are a lot of contradictions in what Trump said about them during the campaign that haven’t been resolved yet.”

Today, there are almost 2,000 deployed nuclear weapons in the US arsenal, riding on submarines, sitting atop missiles, or packed into bombs, according to the Federation of American Scientists, and another 5,000 or so are in the military stockpile. Russian numbers mirror the US ones.

“We still have more than enough nuclear weapons to trigger a full-scale nuclear winter that would destroy food production worldwide,” atmospheric scientist Alan Robock of Rutgers University told BuzzFeed News. “It would be a horrible holocaust that destroys humanity.”

During the campaign, Trump raised alarm among experts in the nuclear establishment by promising to dismantle a 2014 nuclear deal with Iran and shrugging over South Korea and Japan building their own nukes. In August, MSNBC anchor Joe Scarborough cited an anonymous source claiming that Trump repeatedly asked foreign policy experts why the US can’t use its nuclear weapons.

The last claim triggered a denunciation of Trump by former Mitt Romney national security adviser John Noonan, who Tweeted: “Trump would be undoing 6 decades of proven deterrence theory. The purpose of nukes is that they are never used.”

In a debate the next month, Trump said that in a war, “I would certainly not do first strike,” but later walked back the comment, saying “I can’t take anything off the table,” triggering still more confusion. Leaving the question of a first strike open has been a bipartisan facet of US nuclear diplomacy for decades, meant to discourage Russia and China from adventurism.

“I hate proliferation. I hate nuclear more than any,” Trump told CNN later in the campaign when asked about some of these conflicting statements.

“It is very hard to forecast anything with respect to Mr. Trump, as his views appear to be very shallow and often contradictory,” William Potter of the Middlebury Institute of International Studies at Monterey, California, told BuzzFeed News by email. “Much will depend on the national security team he assembles.”

When assembling that team, Potter said, Trump’s options may be limited. In August, 50 Republican military advisers — including many veterans of the George W. Bush administration — released a letter denouncing Trump’s candidacy.

If it follows the lead of previous administrations, the Trump team will first conduct a “nuclear posture” review, headed by a new Secretary of Defense and National Security Advisor. The review will set the outlines of Trump’s approach to building nuclear weapons and seeking to limit their proliferation worldwide.

Until then, it’s anybody’s guess on his plans. But here, in no particular order, are the biggest issues he’ll face.

1. The Iran deal

Fabrice Coffrini / AFP / Getty Images

US Secretary of State John Kerry and other world leaders at the announcement of an agreement on Iran nuclear talks on April 2, 2015.

Trump said he would immediately “renegotiate” the United Nations security council agreement with Iran, which in 2014 traded relaxed economic sanctions for a halt to that nation’s efforts to make an atomic bomb.

“My number one priority is to dismantle the disastrous deal with Iran,” Trump said in March during the heat of the campaign. “We got nothing in return,” he said.

But if the US re-imposed economic sanctions on Iran in a bid to renegotiate the agreement, despite its compliance on halting nuclear weapons work, the only result would be Iran resuming its bomb-making without facing any economic sanctions from Russia or Europe.

“Walking away from the treaty would be an enormous mistake,” Hans Kristensen of the Federation of American Scientists told BuzzFeed News.

If the deal is scrapped, the only real way to stop Iran from making weapons is to threaten war, Pomper said.“And if you threaten military action, you have to be willing to follow through,” he added. “I’m not sure anyone voted for another war in the Middle East.”

2. Stopping North Korea

Chung Sung-jun / Getty Images

South Korean protesters hold anti-North Korea placards displaying portraits of North Korean leader Kim Jong-Il and a model of a nuclear missile.

North Korea, which has tested atomic bombs and is developing missiles to carry them, is also a problem. Trump’s shocking idea that South Korea might need to develop its own nuclear forces as a counter raised alarms, but his May suggestion that he would sit down and talk directly to North Korea’s Kim Jong Un, despite a lack of formal diplomatic relations with the epically unreliable Hermit Kingdom, might also open new possibilities.

“It could be a complete disaster,” Wright said. “But maybe it could be useful.”

3. Paying for all of our weapons

Trump’s big-ticket challenge is whether to continue the costly overhaul of US nuclear weapons — subs, bombers, and missiles — that the Obama Administration has pursued, estimated at more than a trillion dollars over the next 30 years. On top of that, there’s the $9 billion cost of 1,000 extra nuclear cruise missiles that the US Air Force would like.

“Trump might want to ask why all the other nuclear powers, except the US and Russia, only have a few hundred bombs, and if we really need that many,” Robock said.

4. Dealing with Russia

Another nuclear headache for Trump’s team comes from Russia, which is reportedly making medium range nuclear cruise missiles that violate a Reagan-era arms control treaty. Trump has promised better relations with Vladimir Putin, which could mean that Russia will cooperate more with Trump once he takes office.

“There’s always the chance that Republicans can make agreements with Russia that Democrats can’t,” nuclear arms analyst David Wright of the Union of Concerned Scientists told BuzzFeed News. There’s much less political incentive for Republicans in Congress to attack a Republican President over a deal, unlike Obama.

Wright pointed to disarmament treaties with Moscow from the last two Bush administrations, as examples. “Trump has promised a rethinking of our relationship with Russia.”

5. Putin as puppet master

The sheer number of US and Russian warheads makes any change in Trump and Putin’s relationship the most paramount in the US nuclear enterprise, said a number of experts. “You could see the Russians saying ‘maybe we could give up something small to work with this guy’, in exchange for more freedom in the Ukraine,” Pomper said.

In the past, however, inexperienced presidents have ended up deferring to military leaders who tend to see nuclear diplomacy in stark terms, leaving little room for breakthroughs or the kinds of negotiations bruited by Trump during the campaign. This was a criticism that nuclear disarmament advocates made of Obama, Wright said, throughout his presidency.

“The biggest riddle,” said Potter, is whether Trump’s team will clash with a “very hawkish group of Cold Warriors” at the US Department of Defense and State Department who would resist easing up on Russia.

“My guess is that the Cold Warriors ultimately will prevail, and we will soon sink back into an even more expensive and ideologically charged arms race with Russia,” Potter said. Congress, he pointed out, has been eager to spend money on the military, breaking a budget agreement with Obama that had somewhat restrained spending in recent years.

“I suspect that Moscow’s celebratory welcome of the election outcome, bordering on intoxication, will soon morph into a much darker hangover.”

Dan Vergano is a science reporter for BuzzFeed News and is based in Washington, DC.